In coating moving surfaces, such as paper webs, certain defects are produced if the coating is not applied uniformly by the coating blade. Such coating flaws or linear flaws may be produced by an obstruction in the coating nip, dilatency of the coating or coating inconsistency.
The present decection methods used are primarily the observations of the machine operators. This is totally unsatisfactory in that in the case of a paper web, it normally would move as fast as 2,000 ft. per minute and it is very difficult to detect and report coating defects. Electronic devices to detect flaws are available. One such device includes shoulder-to-shoulder silicon detectors. The detectors may or may not scan a predetemined portion of the width of the web. The sensing elements of such detectors generally provide an incident beam such as from a light emmitting diode and the reflected beam is collected by a photocell. When the beam of light traverses a gloss variation, the reflected signal is greatly reduced. The photocell senses this light level change at the specular angle, automatically compensating for gloss variations, and an output signal is provided.
In coating paper webs, ideally a uniform coating is applied, resulting in uniform gloss over the entire coated surfaces. However, in practice in addition to the coating flaws described above other variations in gloss appear such as caused by variations in formation, flocculation, platelets, etc. Generally, it is only desired to detect linear flaws which require rejection of the coated web while overlooking variations in gloss which are not serious enough to require rejection of the coated web.
The problem with sensing elements that detect variations in gloss is that variations relate not only to linear flaws, but also to most other variations in the paper stock being coated, and in the coating.
It is important therefore to provide a sensing unit which would detect only those flaws which determine whether or not the coating being applied warrants that the coated stock be accepted or rejected. Thus, of the flaws which occur in a coating, rejectable flaws must be distinguished from non-rejectable flaws. Present sensors are not able to accomplish this since they "see" all flaws which results in an unacceptably high rate of rejection. That is, in an automatic inspection system, there is a tendency to report all variations in gloss as flaws.